Cryosurgical probe

Abstract: A cryosurgical probe comprises a probe head operable to be cooled by the expansion of a refrigerant gas within the probe head; a probe handle having means for precooling the refrigerant gas; and a flexible catheter linking the probe handle and the probe head, the catheter defining a channel for carrying precooled refrigerant gas from the probe handle to the probe head.

Description: This invention relates to cryosurgical probes. Cryosurgical probes employing a cooling element disposed at a distal end of a flexible catheter are used in the treatment of internal bodily organs such as the heart. An example of a cryosurgical probe is disclosed in the British published patent application number GB-A-2 226 497. This probe comprises a handle portion, a flexible catheter and a probe head. The probe head may be cooled by the expansion of a refrigerant fluid within a cavity of the probe head. In use, the probe head and catheter are inserted into for example, a patient's blood vessel such as the femoral vein, and are steered so that the probe head occupies a position within the heart. High pressure refrigerant gas is then supplied through the catheter to the probe head to cause cryosurgical necrosis of small areas of the cardiac tissue which are responsible for malfunctions such as cardiac arrhythmias. GB-A-2026324 discloses cryotherapy apparatus which is arranged to prevent excessive cooling of the probe handle. A temperature sensor senses the temperature of the handle. If the handle temperature is too low, a valve is operated to restrict refrigerant flow to the probe. Viewed from a first aspect, this invention provides a cryosurgical probe control unit for controlling a cryosurgical probe having a probe head operable to be cooled by the expansion of a refrigerant fluid within the probe head, the unit comprising: means for supplying a refrigerant fluid to the probe head; characterized in that the unit further comprises: means for detecting a sudden increase in the probe head temperature; and means, responsive to the detection of the sudden increase in the probe head temperature, for preventing and venting the flow of the refrigerant fluid to the probe head. Viewed from a second aspect, this invention provides a cryosurgical probe comprising: a probe head operable to be cooled by the expansion of a refrigerant fluid within the probe head; a probe handle; and a flexible catheter linking the probe handle and the probe head, the catheter defining a channel for carrying the refrigerant fluid from the probe handle to the probe head; characterized in that the probe further comprises: means for detecting a sudden increase in the probe head temperature; and means, responsive to the detection of the sudden increase in the probe head temperature, for preventing and venting the flow of the refrigerant fluid to the probe head. The present invention improves safety because a sudden increase in temperature (e.g. an increase to at least a predetermined temperature) can indicate an increase in the fluid back pressure (e.g. caused by a blockage), and it is safer to shut off and vent the fluid flow in these circumstances. The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which: Figure 1 is a schematic diagram of a cryosurgical probe; Figure 2 is a schematic diagram illustrating the cooling operation of a probe head; Figure 3 is a schematic diagram illustrating the precooling operation of a probe handle; Figure 4 is a schematic diagram illustrating a probe handle connected to a control apparatus; Figure 5 is a schematic diagram illustrating steering of the probe head by torque control; and Figure 6 is a schematic diagram of a two-axis steering mechanism. Referring now to Figure 1, a cryosurgical probe comprises a probe handle 10, a probe head 20, and a flexible catheter 30 linking the probe handle and the probe head. The cryosurgical probe shown in Figure 1 may be used in surgical procedures in which a patient's bodily tissue is locally cooled to such a degree that the tissue is locally destroyed. The cryosurgical probe has particular application to treatment of internal organs such as the human heart. In this case, the probe head 20 and the flexible catheter 30 are inserted into the femoral vein from a position in the patient's groin, and are passed through the patient's vein structure to reach the heart. Once the probe head 20 is in position in the patient's heart, the probe head can be cooled to destroy small portions of the heart tissue responsible for malfunctions of the heart such as arrhythmias. The cooling of the probe head is performed using expansion of a refrigerant gas in accordance with the Joule-Thomson effect, described in the book 'Equilibrium Thermodynamics' (C J Adkins, Cambridge University Press, 1983). For this purpose, the refrigerant gas at a high pressure (e.g. 4 x 10 6 Pascals) is supplied to the probe handle 10 via an inlet tube 40, and exhaust (expanded) gas at a lower pressure is returned from the probe handle 10 by an exhaust tube 50. The probe handle 10 comprises means for precooling the high pressure refrigerant gas to be used for cooling at the probe head. This precooling can improve the cooling performance of the probe head. The means for precooling will be described in detail below. The flexible catheter 30 has a diameter of nominally 3 mm (conventionally referred to as a '9 French catheter'). The outer wall of the catheter 30 is strengthened with a metal braid, which is also used as an electrical conductor (see below). Figure 2 is a schematic diagram illustrating the cooling operation of the probe head 20. In the probe head 20, high pressure refrigerant fluid is delivered through a narrow bore tube 60, and expands from the end 70 of the tube 60 into a larger exhaust cavity 80. This causes local cooling of a rounded metal tip 90 of the probe head by the Joule-Thomson effect. The rounded metal tip 90 of the probe head 20 provides a smooth leading surface for the probe head 20 as it is directed along a patient's blood vessels, and also allows effective heat conduction from the patient's tissue in contact with the tip 90 to the expanded refrigerant gas in the probe head 20. Figure 3 is a schematic diagram illustrating the precooling operation of the probe handle 10. High pressure refrigerant gas received through the inlet tube 40 is passed through an axial conduit 100 in the probe handle 10 before entering an axial refrigerant supply tube 110 in the catheter 30 (connected to the tube 60 in the probe head 20). The refrigerant supply is a polyamide tube having a thermal conductivity of less than 1 Watt per metre - Kelvin (W/mK) and a rigidity modulus (EI) of less than 20 Newtons per square metre (N/m 2 ). An aperture 120 in the conduit 100 allows a portion (about two thirds) of the refrigerant gas to expand into an expansion area 130 of the probe handle 10. As mentioned above, this causes the expanded refrigerant gas to cool by the Joule-Thomson effect. The cooled gas is then directed by a helically-vaned heat exchanging structure 140 in a helical path around the conduit 100. This allows heat transfer from the high pressure refrigerant gas in the conduit 100 to the expanded gas in the expansion region 130, thereby precooling the high pressure refrigerant gas in the conduit 100. In order to assist the heat transfer, the conduit 100 and the vaned heat exchanger 140 are fabricated from a good heat conducting material such as copper. A second aperture 150 allows the refrigerant gas which has expanded through the aperture 120 to escape along the exhaust tube 50. The portion of the high pressure refrigerant gas which does not expand through the aperture 120 passes into the refrigerant supply tube 110 in the flexible catheter 30, to be supplied to the probe head 20. Exhaust gas from the probe head 20 returns to the probe handle through an exhaust return tube 160 which communicates with the exhaust cavity 80 in the probe head. This exhaust gas from the probe head 20 passes through apertures 170 into an outer region of the body of the probe handle 10. and from there into the exhaust tube 50. Figure 4 is a schematic diagram illustrating the probe handle 10 connected to control apparatus 200, 210. The control apparatus 200 attends to the control of the flow of high pressure refrigerant gas to the probe handle 10 and, ultimately, to the probe head 20. The apparatus 200 comprises a vessel 220 containing high pressure refrigerant gas and connected, via a flow valve 230, to the inlet tube 40. The exhaust tube 50 from the probe handle is connected to an atmospheric air vent 240 or scavenging system. A thermocouple temperature sensor is provided in the probe head 20. An electrical signal from the thermocouple sensor is passed, via signal wires (not shown) within the flexible catheter 30 and the exhaust tube 50 to a temperature detector 250 within the control apparatus 200. An output electrical signal from the temperature detector 250 is passed in parallel to a high pass electrical filter 260 and a feedback temperature controller 270. The high pass filter 260 detects a sudden increase in the temperature of the probe head 20 (e.g. an increase over a threshold tip temperature such as -65° Celsius). Such a sudden increase indicates a corresponding increase in the back pressure (exhaust pressure) of the cooling operation in the probe head 20 and can therefore indicate a possible blockage in the exhaust return tube 160 or the exhaust tube 50. In this case, for safety reasons the flow of refrigerant gas to the probe handle 10 is immediately terminated by the high pass filter 260 applying a control signal to the flow valve 230. The refrigerant supply tube 110 is also vented by an atmospheric air vent (not shown). The feedback temperature controller 270 responds to either a variable temperature control (to be set by a surgeon or other operator using the cryosurgical probe) or, as shown in Figure 4, to three possible temperature selections, namely 'Off', '0 Degrees' and 'Freeze'. In Figure 4, these selections are made by control buttons mounted on the probe handle 10. However, in other embodiments, the temperature controls could be part of the control apparatus 200. When the temperature control is set to 'Off'. the flow of refrigerant gas to the probe handle 10 is shut off completely by means of a control signal from the feedback temperature controller 270 to the flow valve 230. The refrigerant supply tube 110 is also vented to an atmospheric air vent (not shown). When the temperature control is set to '0 Degrees', the feedback temperature controller 270 varies the flow of refrigerant gas to the probe handle 10 using negative feedback in order to maintain a probe head temperature of substantially 0° Celsius. When the temperature control is set to 'Freeze', the feedback temperature controller 270 controls the flow valve 230 to open fully, thereby decreasing the probe head temperature to a temperature (for example, -30° Celsius to -70° Celsius) suitable for cryosurgical necrosis of the patient's bodily tissue. The reason for the three stage temperature control in this embodiment is as follows. During, for example, a surgical operation to eliminate cardiac arrythmia, it is necessary to destroy small parts of the heart tissue responsible for generating or transmitting spurious electrical signals within the heart which cause the arrythmia. However, it is important that incorrect parts of the heart tissue are not inadvertently destroyed. In order to identify the correct portions of the tissue to be destroyed, the surgeon positions the probe head 20 at an approximately correct position within the heart, and then controls the probe head temperature to be reduced to substantially 0° Celsius. This has two effects: the metal tip 90 of the probe head 20 is frozen to a particular portion of the tissue, and electrical activity in that tissue is rendered inactive (although the tissue is not killed) by being cooled to the freezing point of water. If the tissue to which the tip 90 is currently frozen is responsible for the cardiac arrythmia, then cooling that tissue to 0° Celsius will cause the arrythmia to be temporarily stopped. In this case, the surgeon can then operate the control to cause the probe head temperature to be reduced to a suitable temperature for cryosurgical necrosis of that area of tissue. The necrosis of the tissue is then performed without the probe head temperature rising above 0° Celsius, so that the tip 90 of the probe head 20 remains in contact with the same portion of tissue throughout the necrosis process. If, however, cooling the probe head to 0° Celsius does not cause the arrythmia to cease, the surgeon knows that he had not yet identified the correct portion of cardiac tissue to be necrosed. In this case, the cooling of the probe head is stopped and, when the tip 90 has thawed and been freed from the tissue to which it was frozen, another area of cardiac tissue can be tested by cooling to 0° Celsius. The control apparatus 210 in Figure 4 comprises a signal amplifier 280 and a signal display 290. The signal amplifier 280 receives electrical impulses from electrodes disposed at the probe head 20 and amplifies those impulses for identification on the signal display 290. This provides a further aid to the surgeon to assist in correctly positioning the probe head 20 on the area of cardiac tissue to be destroyed. Figure 5 is a schematic diagram illustrating steering of the probe head within the patient's blood vessels by torque control. In this embodiment, the probe head 20 is disposed at an angle to the flexible catheter 30. This means that axial rotation 300 of the flexible catheter 30 (for example by rotating the entire probe handle 10) causes a corresponding change 310 in the orientation of the probe head 20. Figure 6 is a schematic diagram illustrating a two-axis steering mechanism for steering the probe head 20. In Figure 6, two control wires 320, 330 are connected to a rotatable crank 340 forming part of the probe handle 10. The control wires 320, 330 pass along the flexible catheter 30 into the probe head 20 and are linked to opposite sides 350, 360 of the probe head 20. This arrangement means that rotation of the crank 340 in, for example, a clockwise direction causes the control wire 320 to be pushed towards the probe head 20 and the control wire 330 to be pulled from the probe head 20. This in turn causes a downward movement 370 of the probe head 20. This type of steering mechanism can be applied in two orthogonal directions, to provide a four-axis steering mechanism. Figure 6 also illustrates the thermocouple 400 connected via signal wires 410 to the temperature detector 250 in the control apparatus 200. Sensing of electrical impulses at the probe head is provided by two electrodes, one of which is the probe head tip 90 and the other which 420 is an annular metal ring around the probe head 20. The probe head tip 90 is connected to the probe handle 10 by a metal braid which is also used for strengthening the flexible catheter 30. The electrode 420 is connected to the probe handle 10 by a signal wire 430. This arrangement is similar to a coaxial cable and provides screening of the signal wire 430. In another embodiment, four or more electrodes (for example three annular electrodes plus the probe head tip 90) could be used.

Claims: A cryosurgical probe control unit (200) for controlling a cryosurgical probe having a probe head (20) operable to be cooled by the expansion of a refrigerant fluid within the probe head, the unit comprising: means (40, 220) for supplying a refrigerant fluid to the probe head (20); characterized in that the unit (200) further comprises: means (250, 260) for detecting a sudden increase in the probe head temperature; and means (230, 260), responsive to the detection of the sudden increase in the probe head temperature, for preventing and venting the flow of the refrigerant fluid to the probe head. A cryosurgical probe comprising: a probe head (20) operable to be cooled by the expansion of a refrigerant fluid within the probe head; a probe handle (10); and a flexible catheter (30) linking the probe handle (10) and the probe head (20), the catheter defining a channel (40, 110) for carrying the refrigerant fluid from the probe handle to the probe head; characterized in that the probe further comprises: means (250, 260, 400, 410) for detecting a sudden increase in the probe head temperature; and means (230, 260), responsive to the detection of the sudden increase in the probe head temperature, for preventing and venting the flow of the refrigerant fluid to the probe head (20). A probe control unit or probe according to Claim 1 or 2, in which the refrigerant fluid is a refrigerant gas.

Inventors: REEVES SIMON RICHARD, VARNEY KELVIN JOHN, REEVES, SIMON RICHARD, VARNEY, KELVIN JOHN
Applicants: SPEMBLY MEDICAL LTD, SPEMBLY MEDICAL LIMITED
Application year: 1994
Main classification: A61B18
IPC codes: A61B17, A61B18